Centrifugal pump impeller balancing process



JuBy 6, 1943;. P. H. WILLIAMS .CENTRIFUGAL PUMP IMPELLER BALANCINGPROCESS Filed Aug. 4, 1940 INVENTOR Patented July 6, 1943 OENTRIFUGALPUMP IMPELLER BALANC-' ma raocsss Philip H. Williams, San .liose, Calif. 7

Application August 41, 1940, Serial No. 351,350

2 .Qlalms.

In the design and construction of high speed rotative machines,specifically centrifugal pumps, it is imperative that rotating parts beaccurately balanced to eliminate vibration, friction, excessive powerconsumption and rapid wear on packings and bearings. Quiet operation isoften of extreme importance in the operation of pumps, and this isimpossible without accurate balance of the rotating parts.

The usual practice in balancing centrifugal pump impellers is to balancethe finished impeller for either static or dynamic balance, treating itsimply as a piece of metal. Were it in'truth simply a piece of turnedmetal this process would be fairly accurate. But the process in usetoday is inaccurate and inadequate in the balancing of centrifugal pumpimpellers for the following reasons:

A centrifugal pump impeller, as usually con structed, is not a solidpiece of metal but a series of curved metal vanes separated by waterchannels or passages, and shrouded on each side by a thin metal wall.

As the interior of the impeller is cast from sand molds or cores, whichin turn are supported during'the casting process in an outer sand mold,it will be evident that such inner cores or moulds may shift, deform, ordistort under the action of the heated metal and accumulated gases. Evenif accurately made patterns and core boxes are used and the utmost careexercised, some distortion takes place from uneven cooling, uneventamping of the sand, uneven density of the cores'and uneven shrinkageduring cooling of the castings.

After the castings reach the machine shop it becomes all but impossible,and definitely impractical, for the machinist to determine the exactsymmetrical center of the rough casting. The result is that when theimpeller is finished and comes from the lathe it is certain that thevanes and water channels are not symmetrically disposed about the'truecenter of the impeller as it was laid out on the drafting board.

Again, the wall thickness of the shrouds will vary at several pointsfrom reasons stated above, and the metal of the finished impeller willbe lighter or denser at various points.

From what has been written it will appear:that the present method ofbalancing an impeller ignores the symmetry of the design and proceeds aswith solid metal. In other words the balancing is done without regard tothe weight of the liquid to be pumped or the relation of such Weight tothe design center of the impeller.

From what has been written it will appear that when actually pumpingWater the weight of the water or liquid in the impeller will not besymmetrically disposed about the axis of rotation of racies and varyingthickness and density of the cast metal. My invention is designed tomake possible an accurate balance of pump impellers under the conditionsdescribed.

This invention relates to the balancing of centrifugal pump impellersand the like, and more specifically, to the balancing of cast impellersto insure accurate balancing under actual operating'conditions.

An object of the invention is to provide a process and a means ofbalancing a centrifugal pump impeller so that when filled with theliquid it is designed to pump, it will be in accurate balance regardlessof inaccuracies of construction and lack of symmetry in the vanes andchannels of the impeller.

Another object of the invention is to provide a 7 process and a means ofbalancing such impellers for the various liquids handled in industry andto adapt the balance of the impeller to the specific liquid to bepumped.

Still another object of the invention is to provide such means for thepurposes named as shall be low in cost, simple in operation and ofpractical application to the problems of balancing centrifugal pumps ina pump manufacturing plant.

I attain these objects by use of the process described in theaccompanying specification and illustrated and made clear by referenceto the annexed drawing, in which Figure 1 is a side View of an impellerfrom the suction side and showing a balancing ring in place.

, channels of the impeller.

Similar numerals refer to similar parts throughout all the figures ofthe drawing.

Referring to the accompanying drawing,

l is a shroud or side plate of the impeller.

2 is a balancing ring used as hereinafter described.

3, 3, 3 are impeller vanes, shown in section in Figure 2.

4 is the hub of a single suction impeller.

5 is the suction nozzle of a single suction impeller.

6 is a balancing arbor or the shaft of. the imexact size or symmetry orsymmetrically disposed about the mechanical center I.

With these details clearly in mind it will appear that if, the impellercould be filled with the liquid to be pumped, and the liquid retained inplace during the balancing process, then in that event the impellerwould be in balance when actually pumping the liquid. It will also beevident that the same results would be attained could we use somesubstanceof equal weight that would retain its position during thebalancing process.

I have discovered that there are certain waxes, or that certain waxescan be produced, that will have the same specific gravity of the liquidto be pumped,'be that liquid water, milk, brine, oil, etc.

Referring to the drawing, I press a. balancing arbor 6, through theimpeller as in conventional I balancing arrangemenathen a metal ringwith a light push fit and wide enough to close the outer ports of theimpeller, is pressed by hand over the outer circumference of theimpeller closing the outer ports. It will be evident that the ring initself is in balance, being turned inside and outside from solid metal.The impeller is now placed with the balancing arbor 6 vertical and withthe suction nozzle 5 up and theheated balancing wax is poured into thenozzle until the channels and openings in the impeller are completelyfilled. 4

The wax, being heated only to melting temperature rapidly solidifies andthe impeller may be balanced either statically or dynamically withoutdisplacing the wax. The balancing ring 2 securely holds the balancingmedium against displacement by centrifugal force under high rota tivespeed for dynamic balancing.

Two methods, hereafter termed "the convenheavy side of the impeller willrotate the impeller upon its supports until it comes to rest with theunbalance down. By a trial and error method enough metal is removed fromthe heavy side of the impeller to allow the impeller to remain at restin any position.

When the balancing is completed and the impeller readyjor assembly, itis only necessary to warm the impeller to the melting point of the waxwhen the balancing medium will run out into any convenient receptacleand be ready to use again. The usual practice is to press out thebalancing arbor, 6 remove the balancing ring 2 and place the balancedimpeller on a drain rack over a receptacle in a low temperatured oven.In a few minutes the impeller will be completely drained and thebalancing medium all recovered to be used again.

As a balancing medium I use commercial waxes of either vegetable, animalor mineral composition as best suit the problems in hand. Beef or muttontallow with a specific gravity ranging from 0.895 to 0.953; beeswax witha specific gravity of 0.961 to 0.968; carnauba wax with a specifiogravity of 0.995 to 0.999 may. be used when their specific gravitycorresponds with the liquid to be pumped. The carnauba wax at highestspecific gravity of 0.999 is but one tenth of one percent below theweight of water, which renders it accurate enough for practicalpurposes. I have found, however, that reheating and heating to hightemperatures tends to increase the specific gravity of carnauba waxuntil it is possible to approximate the exact weight of the liquid to bepumped.

In special cases I have used successfully beef tallow warmed to workingconsistency and enough finely divided graphite stirred in to give theexact specific gravity desired. This process is practical and there area number of combinations possible to increase the specific gravity ofthe balancing medium. Certain commercial resins have a specific gravitygreater than unity and are used to mix with waxes as, for example,shcemaker's wax is a mixture of beeswax and resin.

From what has been written it will appear that I have provided a lowcost, practical and efficient process for the purposes named, and thatthe balancing medium is low in cost and easily obtainable. It willfurther appear that the balancing process set forth above is new, novel,and

' solves a serious problem in the industrial and tional manner" arecommon in impeller balancing, either of which is applicable to theprocess described herein.

1. Dynamic balancing.The impeller. after being filled with thebalancing-medium is mounted on the spindle of a dynamic balancingmachine and rotated at high' speed. The balancing machine registers theamount of unbalance and at the same time indicates where metal isv to beremoved to restore balance. -Metal is removed by grinding or drillinguntil the desired balance is attained.

2. Static balance-The impeller is mounted on a balancing arbor or shaft,filled with the balancing medium and the ends of the shaft supportedupon parallel ways or knife edges. The

manufacturing world.

Having thus described my invention, I claim: 1. The process of balancingacentrifugal pump impeller and the like, consisting in filling theimpeller passages completely full of a medium of a specific gravityapproximating that of the liquid to be pumped, retaining such medium bysuitable means, balancing the impeller in the conventional manner whilefilled with such medium, and removing such medium by the application ofheat.

2. The process of balancing a centrifugal pump impeller or the like,consisting in filling such impeller or the like with a mediumapproximating the specific gravity of the liquid to be pumped, balancingthe impeller or the like in the conventional manner and removing thebalancing medium by suitable means.

PHILIP H. WILLIAMS.

